Bone screws are used in the medical field for a variety of purposes. Typical uses for bone screws, which may be also referred to as bone anchors and will be discussed as such below, include treating a bone fracture, attaching a corrective device to parts of a fractured bone in an area adjacent to the fracture, and attaching a corrective device to a group of bones. For example, bone screws could be used to attach a plate to one or more vertebrae of a spinal column, such as in one or more of the anterior cervical, anterior or lateral thoracic, anterior or lateral lumbar, anterior lumbosacral or anterior sacral positions.
Bone screws may be used to mount suitable instrumentation—such as clamps, rods, and plates—to bones or other body tissues. Unfortunately, many of the known bone screws can be susceptible to toggling within the body tissue and can also pull out of the body tissue longitudinally due to the substantial forces on the screws from human body movement and muscle memory. So order to achieve a high pull-out resistance, it is known to thread a bone screw all of the way through a bone and place a nut on the opposite side. However, use of such a nut increases the complexity of the surgical procedure and may not be possible in a desired application if access to the opposing side of the bone is limited.
Additionally, known bone screws have a tendency to work free from the bone by rotating in a direction opposite the insertion rotation direction, thereby “backing out” of engagement with the bone and any corrective device through which the bone screw extends. Though this backward rotation differs from pullout due to toggling, the end result is still an undesirable displacement of the bone screw from the bone and/or an associated corrective device, which may necessitate additional surgeries and cause renewed patient trauma.
U.S. Pat. No. 4,484,570, issued Nov. 27, 1984 to Franz Sutter et al. (hereafter referenced as “the '570 patent”) discloses a plate provided with clearance holes and fastening screws (Abstract). The fastening screw of the '570 patent is provided with a slotted clamping part and an expander (Col. 3, lines 21-23). The expander screws into the clamping part, thereby forcing the clamping part to splay outward and become fixedly engaged with the clearance hole (Col. 3, lines 29-32).